Wow it takes three yrs to get vit D levels at optimum? I take 4-5 IU's a day and I way approx 120 lbs. I had my vit D levels last checked three yrs ago and I was told they were low but I don't know what the number was. I started supplementing higher after the last test. Doc said to take 800 a day and my chiro told me to take 4-6 a day. I listened to him (chiro) instead of her (PCP).